The provision of steam to a turbine is typically accompanied by the transfer of heat between the steam and those parts of the turbine coming into contact with the steam. This heat transfer has the tendency to create thermal distortion in various components of the turbine due primarily to thermal expansion and/or contraction occurring as a result of the heat transfer. Any resulting deformation of such turbine parts can be of two types: elastic which is recoverable upon release of the distortion, and plastic which is permanent. In certain application plastic deformation can be significant enough to permanently damage stay bars mounted within the inlet chamber or the horizontal joint flange of the turbine inner casings, requiring costly repairs and replacement of damaged parts.
Thermal distortion could also become significant enough to cause an ovalized deformation of the ends of the turbine inner casing. Such ovalized deformation can cause portions of the inner casing which are in close proximity to the rotor blades to move away from such blades resulting in increased clearance and attendant leakage. A far more serious consequence of such ovalized deformation is where portions of the inner casing move towards the rotor blades. If such movement is significant, the rotor blades will rub the surface of the inner casing causing damage and degradation of efficiency.
It is therefore desirable to minimize the effects of thermal deformation in order to preserve the efficiency as well as the proper alignment of the turbine.
The problems caused by thermal deformation of turbine components are a particularly important consideration in the design of low pressure steam turbines. In low pressure steam turbines there is a significant difference in the steam temperature at the turbine inlet and at the turbine exhaust or annulus. For example, it has been determined that steam entering a low pressure turbine inlet can have a temperature of approximately 700.degree. F., whereas the temperature of the steam as it crosses the last row of blades can be approximately 100.degree. F. The thermal loading resulting from such a temperature drop can cause the above described effects.
Consider briefly a presently available low pressure steam turbine such as that shown in FIG. 1. Steam from a source (not shown) is provided to turbine 10 through conduit 12 which is attached to the outer surface of outer casing 14. Steam passes through an opening in the outer casing, through an opening in an inner casing 16, to an inlet chamber 18, which chamber is formed in the inner casing. A rotor 20 is mounted in bearings 22 to rotate about an axis of rotation "A". A number of annular rows of blades 24 are disposed about the periphery of rotor 20. A number of stationary annular rows of blades 26 are operatively positioned in relation to rotor blades 24 for directing the steam onto rotor blades 24. Stationary blades 26 are positioned through their attachment to various blade rings which in turn are attached to walls 30 of inner casing 16. Inlet blade rings 29a and 29b are positioned such that an opening 31 is formed therebetween for the passage of the flow of steam. Inner casing 16 is aligned to outer casing 14 by fitted dowl assemblies 32.
Inlet chamber 18 is shown to include sidewalls 34 which are oriented at an angle to the axis of rotation "A". Sidewalls 34 are attached at one end to walls 30 and at the other end to inlet rings 29a and 29b. A number of stay bars 36 (only one is shown) are provided between inlet rings 29a and 29b. A number of ribs 38a and 38b are positioned within inner casing 16 about rotor 20, such that the ends of each rib is in contact with walls 30 and inlet rings 29a and 29b.
In operation, a flow of steam is supplied to turbine 10 through conduit 12. The steam passes through outer and inner casings 14 and 16 to inlet chamber 18. Inlet chamber 18 directs the flow of steam to a rotor midpoint where the steam expands axially through alternating annular rows of stationary and rotor blades causing rotor rotation. After crossing the last row of blades the flow of steam is directed through exhaust 39 where it may be recycled.